An Ultra Dense Network (UDN) is typically deployed in highly populated areas such as hot spots, office building, or downtown area at cities, where there are demands of high data rate service. Currently, data traffic is boosting rapidly while there is a clear bandwidth limit in low frequency bands. Hence, it is necessary for UDN to utilize a higher carrier frequency and a wider bandwidth in order to reach an even higher data rate. Accordingly, the UDN is supposed to operate over higher frequency, such as Millimeter-wave (mmW) frequencies ranged from 30 GHz to 300 GHz.
However, at such high transmission frequency, the path loss becomes much higher than that at low transmission frequency. For example, the attenuation for a carrier at tens of GHz (fhigh) is about (fhigh/flow)2 times larger than the attenuation for a carrier at several GHz (flow).
A beacon signal transmitted by an access point (AP) in the UDN can play as synchronization signal, one or multiple preambles for control or data signal detection, beam training preamble, etc, or any combination thereof. For example, by means of the synchronization signal, a user equipment (UE) can detect an AP, synchronize to the AP and receive the control signal so as to access the AP when necessary.
Since the beacon signals play such an important role in the network, all the APs in the network have to transmit them such that the UEs can detect the best AP and request service when necessary, which results in a large overhead for the APs in the network.
In practice, it is a basic policy that the coverage of control channel transmitting the beacon signals shall be larger than or equal to the data channel coverage. In order to overcome the large attenuation in the UDN such that the beacon transmission can meet the desired coverage, the UDN may employ the high gain beamforming antennas. Typically, the beacon signal will be broadcasted by way of beacon sweeping which means that the AP transmits a same beacon signal over a plurality of beams directed to different directions one after another. Here, the beam utilized to transmit the beacon signal is referred to as the beacon beam. As illustrated in FIG. 1, the narrower the beacon beams, the more beacon beams are needed to cover the desired area. In order to ensure best coverage by an AP, a fixed maximum number (N) of beacon beams may be configured by default. Generally, the beacon sweeping is performed periodically. As illustrated in FIG. 2, in each beacon transmission (TX) cycle, all the beams (beam1-beamN) of the AP will be utilized to transmit the beacon signal in the corresponding time slots. Since all the beams are frequently used to transmit the beacon signal, lots of time-frequency resources, which otherwise can be used to transmit the payload data, are occupied by beacon transmission. Such frequent beacon transmission also accounts for the increased antenna power consumption. Furthermore, another disadvantage is that, frequent beacon signal transmission also means frequent emitted interference. From this perspective, it is meaningful to identify the conditions under which transmissions of beacon signals can be reduced.